1. Field of the Invention
The present invention relates to a shape memory alloy which contains Fe, Mn, and Si as basic elements and to a method for producing the same. The shape memory alloy memorizes the shape before plastic working, the strain of which working rs imparted at a Md point or lower temperature. The memory effect appears upon heatrng to an As point or higher.
2. Description of the Related Art
A number of alloys having shape memory properties, from Ti-Ni alloy and Cu-based alloy to Fe-based alloy, have been disclosed (c.f., for example, "Kinzoku", February 1983, page 12). The shape memory effect is a phenomenon accompanying martensitic transformations. Therefore, such alloys respond at a high speed to external force. Further, the identical phenomenon can be utilized repeatedly. Repeated utilization of the shape memory effect is convenient in practical application of the alloys.
The first utilization of a shape memory alloy was for a joint of hydraulrc piping of an airplane. Recently, it has been utilized in broader fields, such as home appliances, industrial robots, engines, and medical devices. For these applications, the shape memory alloy is required to have a particular range of transformation temperatures, the martensite-transformation starting temperature Ms, the austenite-transformation starting temperature As, and the like, hardenability, easy manufacture, workability, and corrosion resistance. For structural uses, the shape memory alloy must have excellent strength, toughness, corrosion-resistance and economicalness
Ti-Ni alloy is exceedingly superior to other alloys in all of these properties, except for easy manufacture and economicalness and has already been put into practical uses. Nevertheless, Ti-Ni alloy has the disadvantage that strict control must be maintained over the ranges of composition of the Tr and Ni, thus preventing mass production. Further, both Ti and Ni are expensive. This limits its usefulness.
Attempts have been made to develop Cu-based shape memory alloys, which are inexpensive. These copper-based alloys, however, are susceptible to intergranular fractures, and suffer from low tensile strength, compression strength, and fatigue strength.
Provisions of an iron-based shape memory alloy with respectively inexpensive alloying elements not only would lead to outstanding advantages, such as the easy manufacture and economicalness, but also would enable improved strength and toughness. These improved properties offered by an Fe-based alloy would enable such structural uses as the fastening parts of a bolt and nut, pipe joints, and functional uses comparable to those of Ti-Ni alloy. It could thus be used in broader fields than Ti-Ni alloy.
Several of Fe-Ni alloys and Fe-Mn alloys displaying the shape memory effect have been reported up to now, but their shape memory ettects cannot be said to be complete. Also they suffer from drawbacks in the range of transformation temperatures and productivity.
Japanese Unexamined Patent Publication (Kokai) No. 53-11861 recites an example of the Fe-Mn alloys. According to this publication, the shape memory characteristic is not appreciable at a Mn content exceeding 30%, allegedly because the magnetic transformation point (.theta..sub.N Neel point) is raised due to a high Mn content and, hence, the .gamma. (face centered cubic structure-austenite)--.epsilon. (closest packing hexagonal structure-martensite) transformation at ambient temperature is suppressed.